It displays the change in numbers of visitors to public pools in the German city of Hanover. The invisible y-axis seems to be, um, nonlinear, but at least it's monotonic, in contrast to the invisible x-axis.

There's a nice touch, though: The eyes of the fish are pie charts. Black: outdoor pools, white: indoor pools (as explained in the bottom left corner).

It's taken from a 1960 publication of the city of Hanover called *Hannover: Die Stadt in der wir leben*.

This is the kind of chart that Ed Tufte made (in)famous. The visual elements do not serve the data at all, except for the eyeballs. The design becomes a mere vessel for the data table. The reader who wants to know the growth rate of swimmers has to do a tank of work.

The eyeballs though.

I like the fact that these pie charts do not come with data labels. This part of the chart passes the self-sufficiency test. In fact, the eyeballs contain the most interesting story in this chart. In those four years, the visitors to public pools switched from mostly indoor pools to mostly outdoor pools. These eyeballs show that pie charts can be effective in specific situations.

Now, Hanover fishes are quite lucky to have free admission to the public pools!